Porous materials are commonly used as membranes, separation media, catalysts, catalyst supports, chemical sensors and a host of other applications. One of the challenges in making porous materials is controlling the pore size distribution. Controlled pore size distribution is essential for the size- and/or geometry-based separation of molecules, ions, or fine particles. A particular pore size within a porous catalyst particle may also favor chemical kinetics of a particular reaction. Clearly it would be advantageous for the porous catalyst particle to have pores all of the desired size to favor the desired reaction and/or separation process.
A research team, C. T. Kresge, M. E. Leonowicz, W. J. Roth, and J. C. Vartuli, developed a synthetic mesoporous crystalline material described in U.S. Pat. Nos. 5,264,203, 5,098,684, 5,102,643, and 5,238,676. The mesoporous crystalline material is made from metal oxide precursors in an aqueous solution of an organic directing agent, for example quaternary ammonium ions. The aqueous solution is prepared by dissolving in water an organic directing agent, for example cetyltrimethylammonium chloride (CTAC) in an amount of about 29 percent by weight (wt %) of the organic directing agent. The aqueous solution is contacted with an ion exchange material for replacement of halide ions with hydroxide ions. Metal oxide precursors are then mixed into the ion exchanged organic directing agent forming a reactive mixture. Metal oxide precursors include sodium aluminate, alumina, silica, and tetraethylorthosilicate. Optionally, a second aqueous solution of tetramethylammonium silicate may be added to the reactive mixture. The reactive mixture is placed in a vessel and heated to a temperature between 95.degree. to 150.degree. C. under autogenous pressure for a time from about a few hours to several days. A solid product in the form of powders or granules is formed and recovered by filtration, optionally rinsed with water, then calcined at about 540.degree. C. for about 1 hour in a nitrogen atmosphere followed by 6 hours in an air environment. The calcined particles exhibit pore sizes in the range from about 13 .ANG. to about 200 .ANG. with variance of pore size from about 15% to about 25%.
Although the method of Kresge et al. produces substantially uniform pore sized materials, the materials are in powder form which is difficult to form into high aspect ratio products. A high aspect ratio product is geometrically three-dimensional and has at least one dimension substantially larger than the other(s). Examples of high aspect ratio products include but are not limited to membranes and coatings. In addition, the method of Kresge et al. involves aqueous phase chemistry which cannot produce certain classes of mesoporous materials from water-sensitive precursors. Water-sensitive precursors, for example metal alkoxides, undergo rapid hydrolysis and condensation in aqueous solution leading to dense precipitated metal oxides or other separate phases which lack substantially uniform mesopores. Although silicon alkoxide precipitates may be re-dissolvable as in the method of Kresge et al., many metal alkoxide precipitates are not readily re-dissolvable under the reaction conditions at which the mesoporous ceramic materials are synthesized. Alkoxide precipitates that are not readily re-dissolvable, specifically for example metal alkoxides, include but are not limited to titanium (IV) ethoxide, titanium (IV) isopropoxide, titanium (IV) butoxide, zirconium (IV) propoxide, and zirconium (IV) isopropoxide, zirconium (IV) butoxide, niobium (V) ethoxide, and yttrium oxide isopropoxide. Other water-sensitive precursors leading to metal oxides that do not readily re-dissolve include but are not limited to many of the metal halides, for example metal chlorides including but not limited to titanium (III) chloride, titanium (IV) chloride, tantalum (V) chloride, zirconyl chloride, niobium (V) chloride, hafnium (IV) chloride, yttrium (III) chloride, scandium chloride, lanthanum chloride, and tin (IV) chloride. Because many metal oxide precursors are water sensitive and because their precipitates are often not substantially re-dissolvable, there are a significant number of metal oxides and multicomponent metal oxide compositions that cannot be formed into a mesoporous, uniform pore-sized product by an aqueous method, for example Kresge et al.
Most ceramic membranes are made by sol-gel and aerogel techniques as summarized by C. J. Brinker and G. W. Scherer, SOL-GEL SCIENCE, (Academic Press, Inc., San Diego, Calif., 1990, pp. 868-870. Particles or chemical precursors are mechanically coated, dried and sintered. Ceramic membranes made in this manner have wide pore size distribution, for example one standard deviation greater than about 15%, random pore arrangement, and random pore shape.
Therefore, it is an object of the present invention to produce high aspect ratio products having substantially uniform pore size distribution.
It is a further object of the present invention to make high aspect ratio products having substantially uniform pore size using a water-sensitive precursor that has a propensity of forming non-dissolvable precipitates in the presence of water.
It is yet a further object of the present invention to form ceramic membranes having substantially uniform pore size distribution.
It is still a further object of the present invention to make substantially uniform pore-sized mesoporous materials using a water-sensitive precursor that has a propensity of forming non-dissolvable precipitates in the presence of water.